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Key to Alnus

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1 Fruits broadly winged; winter buds (sub-) sessile, covered by multiple, imbricate, unequal scales; [subgenus Alnobetula]
1 Fruits narrowly winged; winter buds stalked, covered by 2-3 equal scales.
  2 Pistillate catkins mostly 1-1.5 (-2) cm long, subsessile and often clustered together closely; typical leaves with 8-14 principal veins on each side of the midrib; [subgenus Alnus].
    3 Fruiting catkins drooping; leaves broadest at or below the middle, pale green to glaucous beneath, doubly serrate, the teeth of various sizes, usually some of them coarse; bark dark reddish-brown, shiny, with prominent light-colored lenticels
    3 Fruiting catkins erect; leaves broadest at or above the middle, green beneath, finely serrate, the teeth approximately equal in size; bark light gray or brown, with inconspicuous lenticels
  2 Pistillate catkins mostly 1.5-3 cm long, evidently pedunculate and therefore spaced; typical leaves with 5-8 principal veins on each side of the midrib.
      4 Flowering spring; tree; leaves broadly rounded to slightly notched (emarginate) at the tip; [exotic, uncommonly planted and sometimes persistent in our area, especially northwards]; [subgenus Alnus]
      4 Flowering late summer or autumn; shrub or tree to 9.5 m tall; leaves obtuse to short-acuminate at the tip; [native of e. MD, s. DE, nw. GA, and s. OK]; [subgenus Clethropsis].
        5 Leaf blades ovate, elliptic, or narrowly elliptic (rarely obovate), > 2.1× as long as wide; leaf apex acute (to sometimes acuminate), never obtuse; fruiting strobili > 1.5× as long as wide; large shrub or small tree with broad rounded crown, as wide as or slightly wider than tall; [s. OK]
        5 Leaf blades obovate to elliptic, < 2.1× as long as wide; leaf apex acute to obtuse (to sometimes acuminate); fruiting strobili < 1.5× as long as wide; shrubs or trees taller than wide; [nw. GA,; s. DE and e. MD].
          6 Fruiting strobili (14-) 14.5-19 (-24) mm long, < 1.3× as long as wide; large shrub or tree 5.5-9.5 m tall, with a narrow crown; [nw. GA]
          6 Fruiting strobili (15.2-) 18.5-22 (-25) mm long, > 1.3× as long as wide; medium to large shrub 3.5-6 (-7.5) m tall, with a narrow to broad crown; [s. DE and e. MD]